1. Field of the Invention
This invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. More particularly this invention relates to pre-reacted combinations of additives that improve the adhesion of these organosiloxane compositions to a variety of substrates, including metals and plastics, at curing temperatures below 100.degree. C.
The curing characteristics of organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and the properties of the cured materials obtained from these compositions are desirable for a variety of end use applications, particularly in the electrical and electronics areas, The cured materials are useful as protective coatings, potting materials and encapsulants for integrated circuits and other electronic devices and as coatings for printed circuit boards.
A shortcoming of organosiloxane compositions that cure by a hydrosilation reaction is their inability to adhere strongly to many substrates, particularly plastics and certain metals. One method for remedying this deficiency is by the use of primer compositions or adhesion promoting additives containing silanes and organosiloxanes with a plurality of silicon-bonded hydrolyzable groups and at least one organofunctional substituent that is bonded to silicon through at least one carbon atom. Primer compositions are applied to substrates to which adhesion is desired prior to application of the curable organosiloxane composition. Adhesion promoters are present in the curable organosiloxane composition.
Examples of hydrolyzable groups that are present in known adhesion-promoting organosilicon compounds are alkoxy, carboxy such as acetoxy, ketoximo such as methylethyl ketoximo, and amido. The organofunctional group is typically one that will react with the substrate to which adhesion is desired. Examples of suitable organofunctional groups include but are not limited to alkenyl such as vinyl and allyl, aminoalkyl, epoxyalkyl, mercaptoalkyl and (meth)acryloxyalkyl.
Three classes of silanes frequently used as adhesion promoting additives are epoxy-functional silanes such as 3-glycidoxypropyltrialkoxysilanes, alkenyl-functional silanes such as vinyltrialkoxysilanes and alkyl silicates such as tetraethyl orthosilicate. These silanes can be at least partially replaced by their hydrolysis/condensation products.
European Patent Application No. 240,333 describes organic or silicone resin compositions containing up to 100 weight percent, based on the weight of the resin, of particles obtained by curing an organosiloxane composition containing an alkoxy substituted silane such as 3-glycidoxypropyltrimethoxysilane and an aluminum or zirconium compound.
The use of organic or organosilicon compounds containing epoxy groups in combination with an organic aluminum compound as an adhesion promoter for organosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction is taught in Japanese Patent No. 1,503,369.
The use of an organosilicon compound selected from bis(trialkoxysilyl)alkanes, disilanes containing silicon-bonded alkoxy groups and epoxy-functional alkoxysilanes in combination with a zirconium compound is claimed in European Patent Application No. 493,791, published on Feb. 1, 1993.
The following references teach using two or more organofunctional groups other than alkoxy to achieve cohesive bonding between silicone elastomers cured by a platinum-catalyzed hydrosilation reaction and various substrates. These organofunctional groups can be present on the same or different organosilicon compounds.
The combination of (1) a polyorganosiloxane containing at least one hydroxyl group and at least one vinyl radical and (2) an epoxy-functional silane is taught in U.S. Pat. No. 4,087,585 that issued to Schulz on May 2, 1978. This patent also discloses the necessity of heating curable compositions containing this adhesion promoter to temperatures of at least 100 degrees C. to overcome the inhibiting effect of the adhesion promoter on the curing reaction. The ingredients of the adhesion promoter are blended directly with the other ingredients of the curable composition. The adhesion promoter also extends the working time of the curable composition
Combining the adhesion promoting ingredients of the Schulz patent with a vinyltrialkoxysilane and a chelated organic aluminum compound is taught in commonly assigned U.S. application Ser. No. 980,078, filed on Nov. 23, 1992, now U.S. Pat. 5,270,425.
U.S. Pat. No. 4,732,932, which issued to A. Waldern on Mar. 22, 1988 teaches using as the adhesion promoter a silane containing an epoxy group, at least one alkoxy group and an ethylenically unsaturated hydrocarbon or hydrocarbonoxy radical.
Primers containing an organosilicon compound with aliphatic unsaturated bonds, an epoxy-containing silane and a vinyl acetate polymer are described in U.S. Pat. No. 4,243,718.
The use of polyorganosiloxanes containing an epoxy group and an ethylenically unsaturated hydrocarbon radical as an adhesion promoter for organosiloxane compositions that cure by a hydrosilation reaction is taught in U.S. Pat. No. 4,912,188, which issued to A. Colas et al. on Mar. 27, 1990 and U.S. Pat. No. 4,891,393, which issued to K. Hirai et al. on Jan. 2, 1990. The adhesion promoters of U.S. Pat. No. 4,912,188 were prepared by reaction of an organohydrogensiloxane/diorganosiloxane copolymer with 1) a silane containing an epoxy group and 2) a diene.
Published European Application No. 460,698, published on Dec. 11, 1991, describes organohydrogensiloxanes containing silicon-bonded epoxy groups as adhesion promoters. U.S. Pat. No. 5,082,894, which issued on Jan. 21, 1992, describes similar compositions containing an acetylenic alcohol with at least eight carbon atoms as the catalyst inhibitor and, as the adhesion promoter an alkoxysilane containing an epoxy group and a polyorganosiloxane containing an average of at least one silicon-bonded hydroxyl group and at least one vinyl radical per molecule.
U.S. Pat. No. 4,719,262, which issued on Jan. 12, 1988 to Edwin Plueddemann teaches using reaction products of (1) allyl-substituted organic compounds, including allyl ethers of polyfunctional alcohols and (2) either silanes or disilylalkanes containing at least two alkoxy groups per molecule as adhesion promoting additives or primers. The alkoxy substituted organosilicon compounds do not contain any organofunctional groups.
U.S. Pat. No. 5,106,933 describes adhesion promoters consisting of at least two of (1) an organosilicon compound containing at least one (meth)acryloxyalkyl group, (2) an organosilicon compound containing at least one epoxy and two alkoxy groups bonded to silicon, and (3) an isocyanurate compound.
Canadian Patent No. 1,287,434 describes adhesion promoters containing (1) an organosilicon compound containing diorganosiloxane, triorganosiloxy, SiO4/2 and alkoxy groups, and (2) a silane containing at least two alkoxy groups and either a glycidoxy or epoxycyclohexyl group bonded to silicon.
Primer compositions containing an epoxy-substituted alkoxy- or acetoxysilane, an organic compound containing unsaturated bonds and a tetraorganotitanate are described in Japanese Laid Open Application No. 79/13,454.
Primer compositions containing an epoxy-functional silane, an aliphatically unsaturated organosilicon compound, a vinyl acetate polymer, platinum or a platinum compound, and an ester type organic solvent are described in U.S. Pat. No. 243,718.
The use of hydrolysis reaction products of 1) a tetraalkoxysilane, 2) an organopolysiloxane containing at least one alkenyl radical or silicon-bonded hydrogen atom and 3) an acryloxy-substituted alkoxysilane as adhesion promoters is taught in U.S. Pat. No. 4,786,701, which issued to U. Tanaka on Nov. 22, 1988.
The use of mixtures containing (1) a silane coupling agent with a acrylate or methacrylate group, (2) an epoxyfunctional silane and (3) a partial allyl ether of a polyhydric alcohol in a silicone adhesive that cures by a platinum catalyzed hydrosilation reaction is taught in U.S. Pat. 5,023,288, which issued to K. Hirai et al. on June 11, 1991.
A disadvantage of many organofunctional organosilicon compounds used as adhesion promoters in prior art organosiloxane compositions, particularly the combination of epoxy- and vinylfunctional organosilicon compounds described in the aforementioned U.S. Pat. 4,087,585, is their adverse effect on the cure rate and completeness of cure, particularly at temperatures below about 120.degree. C. Some integrated circuits and other heat-sensitive electronic devices cannot be exposed to temperatures above this limit even for relatively short periods of time without risking damage to the device.
One objective of this invention is to provide a group of reaction products derived from organosilicon compounds that impart adhesion to curable organosiloxane compositions at curing temperatures below about 100.degree. C.
A second objective of this invention is to provide a class of organosiloxane compositions that cure at temperatures below 100.degree. C. to form adherent coatings on heat-sensitive electronic devices and circuit boards containing these devices.